Solar panels are conventionally used as a source of electrical power for spacecraft, such as satellites. The solar panels typically used for spacecraft include a substrate and a plurality of individual photovoltaic solar cells which are secured to a face surface of the substrate. The individual solar cells are electrically connected together to form a series-parallel solar cell array which, when oriented properly toward the sun, converts solar energy into electrical energy.
The most important consideration for solar panels used on spacecraft is reliability. If a solar panel fails in space, it is difficult, if not impossible, to correct or compensate for the resulting loss of electrical power with the result that the useful life of the entire spacecraft is often prematurely ended.
Reliability of solar panels in space applications is difficult to obtain because the solar panels are subjected to a wide variety of conditions in use, many of which are extremely harsh and tend to damage or destroy the solar panels. For example, during the launch of a spacecraft such as a satellite into outer space, the solar panels, which are typically stored for launch in a compact, folded configuration, are subjected to the extreme vibrations and high gravitational forces encountered during blast off. After the satellite has been separated from the launch vehicle and placed into orbit, the solar panels are deployed from their compact, folded configuration and extended to an open configuration with the array of solar cells oriented toward the sun. In the deployed configuration, the solar panels are subjected to substantial thermal stresses; the solar cells and the face surfaces of the substrates are subjected to the intense heat of the sun while the back surfaces of the substrates are subjected to the extreme cold of outer space. Furthermore, in order to have the solar array operate at maximum efficiency it is necessary that the substrate be relatively rigid so that it can maintain all the solar cells in the correct alignment with respect to the sun and be sufficiently strong so as not to break under the forces inherently applied to the deployed solar panels in space. Because of the above-noted requirements it has been extremely difficult to form the solar panels with the required reliability.
The most important consideration after reliability in the construction of solar panels for spacecraft, such as satellites, is weight. The solar panels should be as light in weight as possible to allow increased payloads and increased amounts of fuel to be stored on board the satellite. In this regard, it should be noted that the relative amount of fuel stored on a satellite is particularly important as this fuel is used for the rocket thrusters which are periodically activated to reorient the satellite to the proper attitude. Any additional fuel that can be stored on board by using lighter weight solar panels can be used to extend the useful life of the satellite.
It has not heretofore been possible to construct a solar panel having the desired combination of high reliability and light weight. Suggestions made heretofore to improve the reliability of the solar panels typically increased the weight of the solar panels, and suggestions made to reduce the weight of the solar panels caused a reduction in the overall properties and particularly the flexural strength of the solar panels and thus reduced reliability. For example, it was suggested to use thicker, stiffer substrates and/or to add reinforcement ribs to stiffen the substrate of the solar panels. These proposals and other similar proposals significantly increased the weight of the solar panels. It was also suggested to use thin plastic films as the supporting substrate to lighten the solar panels, but this caused the substrate to be excessively flexible and unstable which decreased its reliability.
State of the art substrates are comprised of a honeycomb core having an outer skin. These substrates are made with various combinations of materials, including aluminum honeycomb cores with aluminum skins, fiber reinforced plastic honeycomb cores with fiber reinforced plastic skins, and aluminum honeycomb cores with fiber reinforced plastic skins. These substrates require, in addition to the honeycomb core and associated skin, reinforcement ribs to provide adequate strength. These ribs, however, increased the total weight. Furthermore, the relatively complex honeycomb substrates are expensive to manufacture and prone to mechanical failure, especially at the junctions of the skin with the honeycomb core.
What would be highly desirable would be a solar panel having a substrate which is relatively strong, simple in construction, has a high degree of reliability, and which is relatively light weight as compared to the solar panel heretofore employed.